In the sector for the design and construction of machines for wrapping stacked products, manufacturers have, over time, used two different construction philosophies, which have led to the definition of two types of machines, respectively automatic and semi-automatic.
These two types originate from a similar basic structure which comprises a support platform for the pallet upon which the group of products to be wrapped rests, and a frame, at the side of the platform, upon which a reel of film for wrapping the group of products is positioned.
In the case of automatic machines, there are solutions which envisage a rotating platform and in which the reel only moves vertically up and down in order to unwind the film along the group of products, or solutions with a fixed platform and a reel which moves vertically and is supported by an arm which rotates around the group of products.
In semi-automatic machines, the first type of system described above for automatic machines is normally used, that is to say, with the rotating platform and a reel which moves in both directions along a vertical axis. These two machines basically differ in that the automatic machines can carry out a series of cycle start and end operations (for example, positioning the free edge of the film near the stack, cutting the film upon completion of wrapping and sealing the edge on the pack thus defined, etc.) using automatic devices and without the direct action of operators on the machine; in semi-automatic machines, on the other hand, operators are required in order to carry out these operations.
An automatic machine for wrapping a load is known from document EP-A-0 630 813. The machine comprises a device for supplying a pre-drawn film, support for the load and a device for handling a free end of the film between two wrapping cycles. The handling device comprises a plate for holding the film against the load and a pair of jaws, located near one of the long edges of this plate, mobile with respect to one another along this edge to crumple and grip between them the portion of film contained between the plate and the film supply device.
Another automatic machine is known from document U.S. Pat. No. 5,138,818, comprising a tumtable for supporting a load to be wrapped, a roll supplying a wrapping material, a wrapping material retaining mechanism mobile from a first position to a second position to retain the wrapping material near the turntable.
A method for cutting film used to wrap a group of products is known from document WO/9906279. The method comprises the step of reducing the width of the section of the film extending between the reel and the pack by means of relative means to define a band of film coplanar with the rotating platform. The method comprises the following steps of gripping the band-shaped section by a gripper and cutting unit attached to the outer edge of the rotating platform on which the pack in placed.
Obviously, these differences mean that the automatic machines are very expensive, used only for high productivity and are top of the range, whilst the semi-automatic machines are economically better suited to companies with medium-small production levels.